The invention pertains to the rare metals metallurgy, namely to the recovery and purification of molybdenum compounds by a gas-phase method.
Molybdenum (Mo) is a malleable transition metal characterized by a high melting point that explains its high heat resistance. Moreover, molybdenum-based alloys have a high specific strength due to their high density. Molybdenum is characterized by a high elastic modulus, a low thermal expansion coefficient, a good heat and corrosion resistance. This metal is resistant in most alkaline solutions, sulfuric, hydrochloric and hydrofluoric acids at different temperatures and concentrations.
Molybdenum is an important local element, taking into account its various oxidation degrees (0, +2, +3, +4, +5 and +6). The metal is a part of a wide range of compounds used in industry.
More than 90% of the world's molybdenum is used as an additive to non-ferrous and ferrous alloys, including steel; the remaining 10% is used for the manufacture of chemical reagents and lubricants. The electrical and electronics industries, as well as military, automotive, and aircraft construction industries use molybdenum as a composite element of steel. Production of inorganic molybdenum-containing dyes, stains and varnishes is another area of application. Molybdenum in trace amounts is increasingly used in fertilizers.
Molybdenum trioxide MoO3 is one of the main molybdenum compounds used in industry. Pure molybdenum trioxide is applied as a laboratory chemical reagent, and commercially-pure molybdenum is used as a catalyst in the petrochemical industry and also as an integral part of ceramic clays, enamels and dyes. Molybdenum compounds are widely used as catalysts or catalysis activators, especially in the petrochemical industry for cracking and reforming of petroleum products and alkylation. Moreover, molybdenum-bearing substances are used for electroplating and etching.
Thus, modern industrial enterprises have a sufficiently high demand for molybdenum in considerable quantities.
The need to invent rational technologies to process refractory molybdenum ores is firstly associated with the reducing raw material base in the molybdenum industry. At the same time, modern industry makes high demands on the purity of materials it consumes.
Production of pure molybdenum compounds requires a number of complex technological processes for mineral ore preparation, its purification and further complex processing. For example, there is a method of producing a high-purity molybdenum. Based on the method, molybdenite concentrates (48-50% Mo) firstly undergo the oxidizing roasting, then the roasted product of MoO3 with impurities is dissolved in ammonia water, and only then the purified (NH4)6Mo7O24.4H2O is extracted from the obtained solution of (NH4)2MoO4. (NH4)6Mo7O24.4H2O is further thermally decomposed to pure MoO3, which is reduced to molybdenum by hydrogen at T=900-1000° C.
Another widely used method described in the same reference is a method of recovering molybdenum by reduction from higher halogen compounds obtained, for example, by a chloride method of processing ore concentrates. The given method includes treatment of oxides with gaseous elemental chlorine in the presence of a reducing agent (carbon) at a temperature above 900° C., separation of chlorination products by condensation and subsequent metal recovery from them. The reduction is carried out either by hydrogen or metallothermically using magnesium, calcium, or sodium as a reducing agent or electrolytically.
The main disadvantage of these methods is high energy consumption resulted from the need to maintain high temperature and high pressure for a long time, as well as complex equipment used and low recovery rate and efficiency.
The Russian patent RU2002839 C1 issued Nov. 15, 1993, describes a method of processing poor molybdenum and tungsten materials through chlorination with gaseous chlorine at a room temperature in the presence of dimethylformamide.
Disadvantage of this method and those described above is incomplete recovery of molybdenum and tungsten from depleted ores, the complexity and energy intensity of the technology and an insufficient purification from impurities, which is due to similar values of temperatures for sublimation of main components in the vapor-gas mixture leaving the reactor. This does not allow efficiently separate all components of the concentrate and therefore to obtain high-purity chlorides, which are starter compounds for the subsequent recovery of pure substances from them. This technology is associated with large amounts of raw materials and auxiliary materials, resulting in a significant amount of various wastes during the chlorination process, including toxic and corrosive gases: elemental chlorine, hydrogen chloride, carbon oxides, phosgene, the neutralization of which is a complex and expensive engineering challenge. In addition, this technology requires expensive reducing agents.
The Eurasian patent EA004480 B1 issued Apr. 29, 2004, discloses a method of selective processing of poor molybdenum and tungsten materials by a continuous-flow method. This method uses selective extraction of tungsten anhydride by low-temperature chlorination using hydrometallurgy. For this purpose, tungsten and molybdenum compounds are transferred to the gas phase using a chlorination reaction at low temperatures close to the boiling point of volatile chloride compounds of recovered metals. Sulfur chloride or sulfur chloride was used in this case as a low-temperature chlorinating agent.
In case of the low-temperature chlorination process that is proposed in the abovementioned patent (maximum temperature of 250-320° C.), tungsten and molybdenum are recovered only in the form of one strictly defined chloride compound: tungsten oxytetrachloride WOCl4 with a boiling point of 240° C. and molybdenum dioxydichloride MoO2Cl2 with a sublimation temperature of 160° C. There is only one associated component—chlorine iron with a boiling point of 320° C.
After the reactor, the obtained vapors of chlorinated oxides were selectively condensed in individual tandem tanks with specific temperatures corresponding to the condensation temperature of each target component. Special nozzles were used to capture each component.
Further, according to the information in EA004480, the obtained chlorinated tungsten product was converted to the target substance by means of hydrolysis in an aqueous medium with the release of gaseous hydrogen chloride which was trapped in an irrigation tower. However, this method is not suitable for molybdenum, since MoO3 does not leach out in alkaline solutions, while hydrometallurgy is a complex and expensive process in this case.
Thus, the main disadvantage of the method described in EA004480 is the hydrometallurgy of rare metals that greatly increases the cost and complexity of the technological process.
Moreover, the given process requires preliminary synthesis of sulfur chloride. There are sulfur-bearing by-products such as SO2, which require a handling system (washing tower, irrigation tower, drop catcher, desiccator, compressor) and subsequent processing.
Another method was also proposed to process molybdenum-bearing raw materials by converting ore components to a gas phase by low-temperature chlorination as described in the Eurasian patent application EA201201076 A1 published on Dec. 30, 2013. In this case, molybdenum trioxide is first reduced to molybdenum dioxide by hydrogen, the obtained product is then granulated and chlorinated with elemental chlorine at low temperature. This technology comprises several stages and involves additional reagents, such as hydrogen, which complicates the technical implementation and increases the cost.
There are several methods for recovering pure molybdenum trioxide MoO3. A hydrometallurgical method is used the most. The method is based on the treatment of the roasted concentrate with ammonia solutions and is usually called the ammonia process. The ammonia process provides for the leaching of the roasted product to produce solutions of ammonium heptamolybdate; the solutions are purified of impurities, and ammonium heptamolybdates are precipitated and subjected to thermal decomposition.
Another popular method of producing pure molybdenum trioxide is sublimation, which is possible due to the high volatility of this compound. Molybdenum anhydride begins to volatilize before melting. Pressure of MoO3 vapors increases significantly at the melting point (795° C.), and there is a sufficiently high rate of evaporation at 900° C. The sublimation process is accelerated with the continuous removal of MoO3 vapors using an air stream or vacuum. The sublimed molybdenum trioxide has a purity of up to 99.975% Mo but is highly dispersed; that often poses difficulties for its further reduction by hydrogen and for its use in industry.
These methods also have disadvantages associated with the need to maintain high temperatures. Moreover, purification as a separate process may be unreasonably expensive and energy intensive.
As can be seen from the above, this field of industry has a need to develop an efficient, fast and inexpensive method of extracting molybdenum of high purity from depleted ores. At the same time, in order to meet the needs of the field this method should be easy to carry out not only in a laboratory, but also at a site with intensive industrial production.